A careful analysis by researchers at several laboratories using a variety of different tests did not find XMRV or related retroviruses in blood samples from people with chronic fatigue syndrome (CFS), according to a report in the September 22, 2011, online edition of Science. The authors of the original report published a partial retraction in the same issue.

In October 2009 Vincent Lombardi and Judy Mikovits from Whittemore Peterson Institute (WPI) in Reno and colleagues first reported -- also in Science-- that nearly 70% of patients with CFS (sometimes known as myalgic encephalomyelitis)had evidence of xenotropic murine leukemia virus-related virus, or XMRV, in their peripheral blood mononuclear cell samples, compared with just 4% of healthy control subjects.

But a series of recent analyses have failed to replicate this finding, suggesting that the reported association might have been attributable to contamination in the laboratory. The May 31, 2011, advance online edition of Science, for example, featured 2 such studies -- one by pioneering HIV/AIDS researcher Jay Levy -- prompting the journal's editor-in-chief to issue an "Editorial Expression of Concern."

In the latest study, members of the Blood XMRV Scientific Research Working Group -- established by the U.S. Department of Health and Human Services (HHS) in the wake of the 2009 report -- compiled coded blood samples from 15 of the subjects (14 of them with CFS) who were previously reported to be XMRV or polytropic murine leukemia virus (P-MLV) positive, as well as 15 healthy donors previously determined to be negative.

Identical sets of samples were distributed in a blinded fashion to 9 laboratories -- including the U.S. Food and Drug Administration (FDA), the Centers for Disease Control and Prevention (CDC), the National Cancer Institute, Abbott Diagnostics, Gen-Probe, and WPI-- which used different assays designed to detect XMRV/MLV nucleic acids, viral replication, and antibodies.

"Researchers used 11 nucleic acid, five antibody, and three culture assays to determine the assays’ abilities to detect XMRV/P-MLVs," HHS explained in a press release describing the findings. "The study also checked for evidence of contamination with mouse DNA, because XMRV or its predecessors may be present in some mouse strains and cell lines."

Two of the labs did see some evidence of XMRV/MLVs, but results were not consistently replicated and reactivity was similar between CFS patients and previously negative controls.

"In summary, our study demonstrates that no XMRV/P-MLV assay in any of the 9 participating laboratories could reproducibly detect XMRV/P-MLV in 15 subjects (14 with CFS) who had previously been reported as XMRV/P-MLV-infected usually at multiple time points and often by multiple assays," the Working Group authors wrote. "The 2 laboratories...that reported positive results in this study reported similar rates of reactivity among XMRV/P-MLV subjects and known negative control donor samples."

The authors of the 2009 report published a terse partial retraction in the September 22, 2011, Science.

"A reexamination by Silverman and Das Gupta of the samples they used shows that some of the CFS peripheral blood mononuclear cell (PBMC) DNA preparations are contaminated with XMRV plasmid DNA," they wrote. "Therefore, we are retracting those figures and table."

However, Mikovits apparently still is not convinced, telling Science writers Jon Cohen and Martin Enserink, "The conclusion of the Blood Working Group was that we don't have a reproducible assay to detect XMRVs in the blood -- not that they weren't in the patients at all.”

Cohen and Enserink explain the long and tortuous history of how retroviruses came to be associated with CFS -- as well as prostate cancer -- in an article in the September 22, 2011, issue, aptly titled "False Positive."

The link has prompted some researchers to explore treating CFS patients with Ampligen, an immune modulator once tried in people with HIV/AIDS prior to the advent of combination antiretroviral therapy. More recently, scientists have tested antiretroviral drugs against XMRV, finding that a few demonstrated some activity.

The initial reports of a link between XMRV and CFS prompted HHS to investigate how well tests could detect XMRV/P-MLV, since if an association had been proven there would be concern that these viruses could put the donor blood supply at risk.

"The results of this study, along with other recent findings, reassure us that these viruses do not pose a threat to the safety of the nation’s blood supply,” stated Susan Shurin, MD, acting director of the National Heart, Lung, and Blood Institute of the National Institutes of Health. "These data add to the mounting evidence that there is no need to screen blood donors for them at the present time."

The HHS press release added that W. Ian Lipkin, MD, at Columbia University’s Mailman School of Public Health is leading a separate study testing fresh blood samples from 150 people living with CFS and 150 healthy controls. Researchers will process, blind, and ship samples to laboratories at FDA, CDC, and WPI to test for the presence of XMRV, MLV, or related viruses.

While research continues, the Blood XMRV Scientific Research Working Group study may prove to be the final nail in the coffin for the XMRV/MLV/CFS hypothesis, disappointing many patients who had hoped finding a viral cause might finally point to effective treatment for their debilitating condition.